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Used biased weak count, cache T* pointer

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Logically, the strong pointer that destroys T owns +1 weak count too
This commit is contained in:
Andrew Noyes
2025-09-11 13:15:03 -04:00
parent f83e21b5a0
commit 10e382f633
1 changed files with 43 additions and 38 deletions
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81
src/reference.hpp
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@@ -24,7 +24,8 @@ struct ControlBlock {
std::atomic<uint32_t> weak_count;
ControlBlock()
: strong_count(1), weak_count(0) {} // Start with 1 strong reference
: strong_count(1), weak_count(1) {
} // Start with 1 strong, 1 weak (biased)
/**
* @brief Increment strong reference count
@@ -64,31 +65,22 @@ template <typename T> struct Ref {
/**
* @brief Get raw pointer to managed object
*/
T *get() const {
if (!control_block)
return nullptr;
constexpr size_t cb_size = sizeof(detail::ControlBlock);
constexpr size_t alignment = alignof(T);
constexpr size_t padded_cb_size =
(cb_size + alignment - 1) & ~(alignment - 1);
return reinterpret_cast<T *>(reinterpret_cast<char *>(control_block) +
padded_cb_size);
}
T *get() const noexcept { return ptr; }
/**
* @brief Dereference operator
*/
T &operator*() const { return *get(); }
T &operator*() const { return *ptr; }
/**
* @brief Arrow operator
*/
T *operator->() const { return get(); }
T *operator->() const { return ptr; }
/**
* @brief Check if Ref is valid (not empty)
*/
explicit operator bool() const noexcept { return control_block != nullptr; }
explicit operator bool() const noexcept { return ptr != nullptr; }
/**
* @brief Destructor - decrements strong reference count
@@ -98,7 +90,8 @@ template <typename T> struct Ref {
/**
* @brief Copy constructor - increments strong reference count
*/
Ref(const Ref &other) noexcept : control_block(other.control_block) {
Ref(const Ref &other) noexcept
: ptr(other.ptr), control_block(other.control_block) {
if (control_block) {
control_block->increment_strong();
}
@@ -110,6 +103,7 @@ template <typename T> struct Ref {
Ref &operator=(const Ref &other) noexcept {
if (this != &other) {
release();
ptr = other.ptr;
control_block = other.control_block;
if (control_block) {
control_block->increment_strong();
@@ -121,7 +115,9 @@ template <typename T> struct Ref {
/**
* @brief Move constructor - transfers ownership
*/
Ref(Ref &&other) noexcept : control_block(other.control_block) {
Ref(Ref &&other) noexcept
: ptr(other.ptr), control_block(other.control_block) {
other.ptr = nullptr;
other.control_block = nullptr;
}
@@ -131,7 +127,9 @@ template <typename T> struct Ref {
Ref &operator=(Ref &&other) noexcept {
if (this != &other) {
release();
ptr = other.ptr;
control_block = other.control_block;
other.ptr = nullptr;
other.control_block = nullptr;
}
return *this;
@@ -142,6 +140,7 @@ template <typename T> struct Ref {
*/
void reset() noexcept {
release();
ptr = nullptr;
control_block = nullptr;
}
@@ -160,11 +159,13 @@ template <typename T> struct Ref {
/**
* @brief Default constructor - creates empty Ref
*/
Ref() : control_block(nullptr) {}
Ref() : ptr(nullptr), control_block(nullptr) {}
private:
explicit Ref(detail::ControlBlock *cb) : control_block(cb) {}
explicit Ref(T *object_ptr, detail::ControlBlock *cb)
: ptr(object_ptr), control_block(cb) {}
T *ptr;
detail::ControlBlock *control_block;
/**
@@ -172,24 +173,19 @@ private:
*/
void release() noexcept {
if (control_block) {
// Prevent weak count from going to 0 concurrently
control_block->increment_weak();
uint32_t prev_strong = control_block->decrement_strong();
// If this was the last strong reference, destroy the object
if (prev_strong == 1) {
T *obj = get();
obj->~T();
}
ptr->~T();
// Now release our temporary weak reference
uint32_t prev_weak = control_block->decrement_weak();
// Release the bias - decrement weak count for strong references
uint32_t prev_weak = control_block->decrement_weak();
// If this was the last weak reference and we destroyed the object,
// we can free the control block
if (prev_weak == 1 && prev_strong == 1) {
std::free(control_block);
// If weak count hits 0, free control block
if (prev_weak == 1) {
std::free(control_block);
}
}
}
}
@@ -219,7 +215,7 @@ template <typename T> struct WeakRef {
expected_strong, expected_strong + 1, std::memory_order_acquire,
std::memory_order_relaxed)) {
// Success - we incremented the strong count
return Ref<T>(control_block);
return Ref<T>(get_object_ptr(), control_block);
}
// CAS failed, expected_strong now contains the current value, retry
}
@@ -314,18 +310,27 @@ private:
detail::ControlBlock *control_block;
// Helper to calculate object pointer from control block
T *get_object_ptr() const {
if (!control_block)
return nullptr;
constexpr size_t cb_size = sizeof(detail::ControlBlock);
constexpr size_t alignment = alignof(T);
constexpr size_t padded_cb_size =
(cb_size + alignment - 1) & ~(alignment - 1);
return reinterpret_cast<T *>(reinterpret_cast<char *>(control_block) +
padded_cb_size);
}
/**
* @brief Release current weak reference and handle cleanup
*/
void release() noexcept {
if (control_block) {
// Prevent strong count from going to 0 concurrently
auto strong = lock();
uint32_t prev_weak = control_block->decrement_weak();
// If this was the last weak reference, check if we need to free control
// block
if (prev_weak == 1 && !strong) {
// If weak count hits 0, free control block
if (prev_weak == 1) {
std::free(control_block);
}
}
@@ -357,6 +362,6 @@ template <typename T, typename... Args> Ref<T> make_ref(Args &&...args) {
}
auto *cb = new (buf) detail::ControlBlock();
new (buf + padded_cb_size) T{std::forward<Args>(args)...};
return Ref<T>(cb);
T *obj = new (buf + padded_cb_size) T{std::forward<Args>(args)...};
return Ref<T>(obj, cb);
}